I need something like \stackrel{\frown}{MM'} to express the fact that MM' is some path. But I am not really satisfied by the look of the result. The \frown is too high. Would you please have a suggestion?

@egreg: Sorry, I didn't notice it. Then it's okey to use yhmath. BTW, \adots in yhmath is wrong for different font size, one should use \iddots from mathdots package instead. That's another reason I don't like yhmath.
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Leo LiuApr 10 '11 at 14:55

Well, and amatrix env. in yhmath is not compatible with latest amsmath. Pity. It should be \newenvironment{amatrix}{\left\langle\begin{matrix}}{\end{matrix}\right\rangle}‌​
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Leo LiuApr 10 '11 at 15:21

I don't know if in practice (I'm not mathematician, sorry) someone will need ever arcs wider than 4 or 5 lowercase characters. If this could be a real scenario, then \widearc of kpfonts, or \wideparen of yhmath or mathdesign, or \bigfrown, probably are not the best solution, since like others extensible accents, are unable to expand too much:

This problem seem well solved with \overparent in Aditya answer, but this require ConTeX. For pdflatex, I think that a decent alternative could be \overgroup from mathabx package: